The purpose of this study is to define electrophysiologic and anatomic bases of abnormal ECG potentials for normal and abnormal cardiac rhythm. The current objectives are to: 1. Define in both the dog and man the locations, actions, and relationships of the multiple pacemakers recently observed in the right atrium. 2. To delineate the interactive roles of abnormal anatomy, conduction, refractoriness, and initiating events in the genesis of atrial and ventricular tachycardias. 3. Deter the bases of left axis deviation in patients with ischemic heart disease and myocardial infarction. The principal techniques used in all studies is the simultaneous recording of cardiac potentials from multiple cardiac sites. In this way multiple sources of excitation as well as complex conduction pathways can be visualized during dynamic states of induced instability. These data will lead to greater understanding of the mechanisms and causes of abnormal rhythm and accompanying electrocardiographic patterns.